1. Field of the Invention
This invention relates to an apparatus for determining a failure of an electromagnetic clutch, and more particularly to an apparatus for determining a failure of an electromagnetic clutch having a solenoid whose coil is energized in a manner such that the amount of current supplied to the coil is feedback-controlled to a desired amount of current.
2. Description of the Related Art
Conventionally, an electromagnetic clutch has been proposed, e.g., in Japanese Laid-Open Patent Publication (Kokai) No. 10-196685 by the present assignee. The electromagnetic clutch has a solenoid comprised of an annular coil of a winding circumferentially wound in layers, an annular coil housing for receiving the coil therein, and an armature arranged close to one axial end of the coil housing. When current is supplied to the coil, a magnetic circuit is formed to attract the armature to the coil housing by an attractive force generated by the magnetic circuit, whereby an engaged state of the electromagnetic clutch is established. Further, by controlling the amount of current supplied to the coil, the density of magnetic flux is varied to change the magnitude of the attractive force of the armature, whereby the engagement force of the electromagnetic clutch is controlled.
Further, it is also known that in order to control the above engagement force of the electromagnetic clutch to a desired engagement force, a desired amount of current (desired current amount) corresponding to the desired engagement force is calculated, and an actual amount of current (actual current amount) actually flowing through the coil is detected, whereby the value of a duty factor of current supplied to the coil is feedback-controlled such that the actual current amount becomes equal to the desired current amount.
In the electromagnetic clutch constructed as above, however, the winding of the coil can be partly short-circuited, causing a so-called xe2x80x9clayer short-circuitxe2x80x9d in which the resistance value of the coil is reduced, e.g., by half. Once the layer short-circuit occurs, even if the actual current amount for the coil is made equal to the desired current amount by feedback control, the density of the magnetic flux is reduced, which hinders the required engagement force from being generated by the electromagnetic clutch. This kind of inconvenience is encountered not only in the case of the layer short-circuit or the like in which the resistance value of the coil is decreased but also in a case where the same is increased for some reason so long as the resistance value is varied to such an extent as permits the actual current amount to become equal to the desired current amount. In this case as well, it is impossible to for the electromagnetic clutch to produce an appropriate engagement force.
It is an object of the invention to provide an apparatus for determining a failure of an electromagnetic clutch, which is capable of properly determining whether or not a failure, typically a layer short-circuit, has occurred due to a change in the resistance value of a solenoid coil of the electromagnetic clutch.
To attain the above object, the present invention provides an apparatus for determining a failure of an electromagnetic clutch including a solenoid having a coil, the electromagnetic clutch being configured such that an engagement force thereof is varied according to an amount of current flowing through the coil of the solenoid, and that an output control value for control of the amount of current is calculated by feedback control such that an actual amount of current actually flowing through the coil becomes equal to a desired amount of current.
The apparatus according to the present invention comprises:
storage means for storing a predetermined reference control value defining a standard for the output control value, the predetermined reference control value having a standard relationship with respect to the amount of current;
actual current amount detection means for detecting the actual amount of current;
comparison means for comparing the calculated output control value with the predetermined reference control value corresponding to the detected actual amount of current; and
failure determination means for determining based on a result of the comparison by the comparison means whether or not the electromagnetic clutch has failed due to a change in a resistance value of the coil of the solenoid thereof.
According to this apparatus, the predetermined reference control value as the standard of the output control value, which has a standard relationship with respect to the amount of current flowing through the coil of the solenoid, is stored in the storage means. Further, the output control value for control of the amount of current supplied to the coil is calculated by feedback control such that the actual amount of current detected by the actual current amount detection means becomes equal to the desired amount of current. Then, the comparison means compares the calculated output control value with the predetermined reference control value corresponding to the detected actual amount of current. The failure determination means determines based on a result of the comparison whether or not the electromagnetic clutch has failed due to a change in the resistance value of the coil.
As the resistance value of the coil is changed, the output control value (duty factor value of pulse current, for instance) mentioned above is calculated by feedback control such that it varies with and compensates for the change in the resistance value of the coil to thereby cause the actual amount of current to become equal to the desired amount of current. Therefore, the difference between the calculated output control value and the predetermined (standard) reference control value corresponding to the detected actual amount of current reflects a degree of the change in the resistance value of the coil. Hence, by comparing the output control value with the predetermined reference control value, it is possible to properly determine whether or not the electromagnetic clutch has failed due to a change in the resistance value of the coil.
Preferably, the apparatus further includes coil temperature detection means for detecting a temperature of the coil, and the comparison means compares a value of at least one of the output control value and the predetermined reference control value, which is corrected according to the detected temperature of the coil, with another of the output control value and the predetermined reference control value.
As the temperature of the coil is changed, the resistance value of the coil is also changed, which changes the amount of current flowing through the coil. Accordingly, the output control value calculated for an identical desired amount of current is changed compared with a case in which the temperature of the coil remains the same. To cope with this, according to the preferred embodiment of the invention, at least one of the output control value and the reference control value is corrected according to the detected temperature of the coil, and then the comparison is made, whereby it is possible to more properly determine a failure of the electromagnetic clutch.
Preferably, the comparison means compares a value of at least one of the output control value and the predetermined reference control value, which is corrected according to a voltage applied to the coil, with another of the output control value and the predetermined reference control value.
As the voltage applied to the coil is changed, the output control value calculated for an identical desired amount of current is changed compared with a case in which the voltage remains the same. To cope with this, according to the preferred embodiment of the invention, at least one of the output control value and the reference control value is corrected according to the voltage applied to the coil, and then the comparison is made, whereby it is possible to more properly determine a failure of the electromagnetic clutch.
Preferably, the apparatus further includes failure determination inhibition means for inhibiting failure determination from being carried out by the failure determination means when the desired amount of current is equal to or smaller than a first predetermined value or when a difference between the desired amount of current and the actual amount of current is equal to or larger than a second predetermined value.
According to this preferred embodiment, execution of failure determination is inhibited by the failure determination inhibition means when the desired amount of current is equal to or smaller than the first predetermined value or when the difference between the desired amount of current and the actual amount of current is equal to or larger than the second predetermined value. When the desired amount of current is small, the output control value calculated based on the desired amount current is prone to variation, and hence an error is liable to occur in failure determination which is carried out by comparing the output control value with the reference control value. Further, the failure determination according to the invention is carried out on the precondition that the actual amount of current has converged on the desired amount of current, and by comparing the output control value calculated in this state of the actual amount of current with the predetermined reference control value. Unless the precondition is satisfied, an erroneous determination of a failure is also liable to occur. Therefore, by inhibiting failure determination in these cases, it is possible to prevent an erroneous determination of a failure of the electromagnetic clutch.
For instance, when the calculated output control value is larger than the predetermined reference control value, the comparison means further compares the output control value with an increased-side reference control value, the increased-side reference control value being obtained by increasing the predetermined reference control value to a predetermined extent, whereas when the calculated output control value is smaller than the predetermined reference control value, the comparison means further compares the output control value with a decreased-side reference control value, the decreased-side reference control value being obtained by decreasing the predetermined reference control value to a predetermined extent.
Preferably, the predetermined reference control value is defined as a value of the output control value to be assumed with respect to the actual amount of current when the coil is under a normal temperature condition, and at the same time a voltage applied to the coil is a normal voltage.
Preferably, the storage means stores the predetermined reference control value in the form of a table of values of the predetermined reference control value correlated to respective values of the actual amount of current.
The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.